This invention relates to the preparation of a water-soluble acylated chitosan finding a wide variety of commercial applications. Specifically, the acylated chitosan is a copolymer of a substance having the formula: ##STR1## and a substance having the formula: ##STR2## wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are independently selected from hydrogen atom and an acyl group, at least one of R's is an acyl group, and m+n&gt;15 and m/(m+n)=0.35 to 0.65. More particularly, this invention relates to a new and improved method for preparing such a water-soluble high molecular weight acylated chitosan within a short reaction time in high yields.
Chitin is a polymer of .beta.-1,4 linked N-acetylated D-glucosamine units each having the formula: ##STR3## Chitosan, a derivative of chitin, is a deacetylated product of chitin and may be obtained by treating chitin with an alkali. At present, these chitin and chitosan are not fully utilized in commercial applications because of their insolubility in water and extremely low chemical reactivity with chemicals. However, water-soluble partially deacetylated chitin obtained by partially deacetylating chitin or water-soluble partially acylated chitosan obtained by partially acylating chitosan can be utilized in various commercial applications as cationic polymers.
One prior art known method for preparing water-soluble partially deacetylated chitin (in other words, water-soluble partially acylated chitosan) starts with naturally occurring chitin. The chitin is hydrolyzed in a homogeneous system at a temperature of lower than about 50.degree. C. while partial deacetylation is effected such that the percent deacetylation of chitin ranges from 40% to 60% (see Japanese Patent Application Kokai No. 53-47479). This prior art method, however, has a number of disadvantages in that (1) it takes as long a reaction time as about 32 to about 98 hours, (2) difficulty in the uniform deacetylation of chitin results in low yields, (3) reaction in the presence of strong alkali at an elevated temperature for an extended period of time results in a relatively low molecular weight product and fails to produce a high molecular weight, water-soluble, partially deacetylated chitin, and (4) preparation of a homogeneous alkali chitin solution is difficult.
In preparing water-soluble partially acylated chitosan by partially acylating chitosan, there may be contemplated a method based on the fact that chitosan is soluble in aqueous acid solution, comprising dissolving chitosan in aqueous acid solution and adding an acylation agent to the solution to effect acylation. Insofar as the present inventors have examined, it is very difficult to produce water-soluble partially acylated chitosan by this method. More particularly, a salt of chitosan with acid is present in the chitosan solution at such a high concentration that acylation readily takes place. Then the degree of acylation of chitosan shortly reaches 95% or higher. As a result, the reaction product becomes water insoluble and the reaction solution becomes gel. For this reason, it is difficult to produce water-soluble partially acylated chitosan by this method.
The influence of the chain length of substituents, degree of substitution, reaction solvent, and other parameters on gel formation in the above-mentioned reaction is reported in the following literature:
Agric. Biol. Chem., 41, 1755 (1977), Hirano et al., PA1 Int. J. Biol. Macromol, 3, 292 (1981), G.K. Hore et al., PA1 Carbohydr. Res., 83, 389 (1980), Fujii et al., PA1 Mcromol Chem., 178, 2595 (1977), Kurita et al., and PA1 Agric. Biol. Chem., 47, 1389 (1983), Hirano et al. PA1 dissolving a water-insoluble chitosan which is a random copolymer of a substance having the formula: ##STR6## and a substance having the formula: ##STR7## wherein a+b&gt;15 and a/(a+b)=0.7 to 1, in an aqueous acid solution,
However, all products obtained by the reactions described in these reports are only improved in solubility in special organic solvents, and they are not water-soluble derivatives.